As is known, comparison reading of the content of a memory cell is performed by comparing a quantity related to the current through the cell to be read with a similar electric quantity related to the current through a reference cell whose content is known. In the case of flash-EEPROM arrays, a similar comparison is frequently also made after erasing the array or part of it, to verify correct erasure and, if necessary, complete the erasure.
According to one solution widely used at present, to safely distinguish between an erased cell and a written cell, the reference cell, when biased to a predetermined read voltage, generates a reference current of a value between that of an erased cell and a written cell. For this purpose, an erased reference cell with a threshold equal to the typical threshold is used, and the reference cell circuitry is so designed that the I-V characteristic (I.sub.DS -V.sub.GS) of the reference cell has a lower slope than that of the array cells. In the above solution, the minimum evaluation (read or verify) voltage is the value at which the current in the erased cell having the maximum permissible threshold is nevertheless higher than the current in the reference cell. This concept is illustrated in FIG. 1 which shows the typical I-V characteristic of a UV, i.e., UV erased, array cell whose threshold depends solely on the fabrication process (curve A); the characteristic of the erased cell with the maximum permissible threshold (curve B); the characteristic of the reference cell (curve C); and the erase-verify voltage (line D), i.e. , the voltage at which erase testing is performed to determine correct erasure of the cells (at the erase-verify voltage, cells with a current I.sub.ds greater than the corresponding reference cell are considered erased). As can be seen, the erase-verify voltage, 3.2 V, is slightly higher than the value at which the characteristic of the erased cell with the maximum threshold intersects the reference characteristic.
According to the above known solution, the erase-verify voltage also represents the minimum read voltage and hence the minimum supply voltage of the memory (which, for economy, is made equal to the read voltage). In the typical case shown, wherein the erased cell and the reference cell both have a 2 V threshold and the erase-verify voltage equals 3.2 V, the maximum-threshold erased cell has a threshold voltage of 2.5 V, and the minimum supply voltage also equals 3.2 V.
On the other hand, current demand is for a reduction in supply voltage as much as possible, to extend application of this type of memory to portable equipment and others requiring low-power consumption.
A reduction in supply voltage, however, cannot be achieved by simply reducing the erase-verify voltage (and hence shifting to the left the characteristic of the erased cell with the maximum permissible threshold), as this would also result in a leftward shift (towards lower values) of the bell describing the threshold voltage distribution of the array cells when erased. Leftward shifting of the bell would also result in the threshold of the more erased cells (left limit of the bell) being brought too close to zero or even to a negative value, thus resulting in depleted cells, i.e., cells which conduct current even when V.sub.GS =0, and which would therefore be read even when not addressed, thus impairing the accuracy of the reading.
Provision must also be made for eliminating as much as possible the presence of cells not yet depleted but with a very low threshold (ranging between 0 and 1 V) which may be subject to hole injection during subsequent programming, thus impairing the oxide layer and hence the reliability of the array.
To at least partially solve the problem of depleted or very low threshold cells, processes are currently employed for recovering the depleted cells. In addition to increasing actual erasing time, however, reducing erase-verify voltage also increases the time it takes to recover the depleted cells, thus considerably increasing erasing time as a whole and reducing the speed of the memory.